Diffusion Lags

First marketing dates are used to determine the diffusion or mean arrival time lags of individual drugs. Following the original launch in the country where a pharmaceutical is first marketed, the drug will spread to other nations. The time difference between the original date and subsequent marketing dates elsewhere provides the basis for calculating the mean arrival time lag. Thus a new therapy that is launched in the UK in 1–61, and spreads to the USA in 1–62, and West Germany in 1–63, will have intervals of 12 months and 24 months. The mean diffusion lag for that drug is the total of the time differences divided by the total number of countries involved. In this example this is (12+24)/3=12 months. An alternative version could be (12+24)/2=18 months where the denominator is redefined as N −1; that is the number of countries less the originating nation. However this procedure has not been adopted. There are arguments for both versions, but neither is obviously a superior measure of the mean arrival time lag. Of course the choice between the two definitions becomes less important for drugs which go to many countries. The average number of destinations reached by a given drug is 9.5, so in fact the overall difference between the two procedures is unlikely to be great. When the mean arrival time lag for each drug has been determined, the ‘all drug’ average is calculated via the total of all arrival time lags divided by the total number of all introductions in destination countries.